The devices that deliver high energy electrical pulses are generally called implantable cardioverter/defibrillators, or ICDs. They generally have two principal elements, a pulse generator, and a lead or a system of leads. The pulse generator (often more simply referred to as a “generator”) functions to monitor the patient's cardiac activity and generate high energy electrical pulses when the heart is determined to have a ventricular arrhythmia that may be treated by applying a shock. As defined herein, the term “shock” or “shock energy” should be understood to mean an electrical pulse of a high energy that is significantly stronger than the electrical energy used for applying conventional stimulations. Such high energy electrical pulse includes a shock for cardioversion and/or defibrillation. The lead or system of leads is connected to the generator and functions to appropriately distribute the shock energy to the patient's heart. EP 0773039 A1 and its counterpart U.S. Pat. No. 5,776,165 assigned to Sorin CRM (previously known as ELA Medical) describes an exemplary generator/lead system, and a technique for selecting an application-specific optimal configuration for delivering required shock energy.
Typically, shock energy is delivered only when it is determined that a true ventricular tachycardia (VT) exists, and not when a supra-ventricular tachycardia (SVT) exists. In the latter case, the SVT is of an atrial origin, and the shock electrode is located at the ventricle, therefore this way of shock energy delivery would be inefficient because the shock energy would be delivered to the ventricle, not to the atrial region. In addition, the application of a shock in a conscious patient is extremely painful and agonizing, because the delivered energy is well above the typical pain threshold. Furthermore, the application of a shock causes side effects on the heart rhythm as it increases a risk of developing secondary disorders, on the functional integrity of the patient's myocardium, and generally, on the patient's physiological balance.
It is therefore desirable to deliver high energy shocks only when appropriate and only if a less painful alternative therapy, such as an appropriate stimulation of the atrium, is not possible.
An analysis of the atrial activity, which implies in particular the recognition of P waves, is a fundamental basis of this technical field. A dual-chamber defibrillator includes circuits for detecting atrial heart rhythm, from which a situation of atrial fibrillation, such as an SVT, is easily detected so as to inhibit the delivery of a shock therapy to the ventricle. However, a single-chamber defibrillator does not have such circuits for detecting rapid activity of the atrium. Thus, if the ventricular rate is fast enough, the device may unavoidably deliver an unintended and inappropriate shock to the ventricle.
However, it is recognized that implementation of a single-chamber defibrillator is sufficient for patients in many cases, especially for those patients for whom a defibrillator is indicated for therapeutic treatment, but whose sinus node has no dysfunction. It should be noted that although implantation of a dual chamber defibrillator is advantageous in improving performance with regard to the classification of tachyarrhythmias (i.e., VF and SVT discrimination), it also is disadvantageous because it increases the risk of complications associated with the relatively greater number of leads and electrodes.
Implantable devices having leads to collect an atrial detection signal using a defibrillation lead are known. For example, U.S. Pat. No. 4,643,201 (assigned to Medtronic), U.S. Pat. No. 5,628,779 (assigned to Pacesetter Inc.) and U.S. Pat. No. 6,321,122 (assigned to Cardiac Pacemakers Inc.) describe various types of leads including a branch or a bend with a specific electrode that is positioned at or in the vicinity of the atrium, once the lead is implanted. EP 0801960 A2 describes another specific type of lead, with a component floating in the atrium, a bipolar electrode pair, a distal component into the ventricle and a distal electrode.
These known implantable devices, however, have relatively complex and specific leads that are not adequate for general use. On the other hand, these leads and electrodes located at the atrium float electrically delivering a relatively noisy atrial detection signal, thus making an analysis of any atrial rhythm difficult.
It is known in the art to connect a generator to a “monobody” lead, which is a single lead that contains various electrodes both to monitor the patient's heart activity and to deliver shock energy. An issue that arises with such a monobody lead is that the collection (also called the detection) of a signal representative of atrial activity is difficult because of signals for noise, e.g., muscular activity and ventricular activity that are also collected and mask the atrial activity component.
EP 1118349 A1 and its counterpart U.S. Pat. No. 6,636,770, assigned to Sorin CRM (previously known as ELA Medical), describes a monobody lead, without any ramification or bent, equipped in its proximal region with two atrial electrodes, two ventricular electrodes, and a supraventricular electrode for the delivery of a shock energy. The atrial signal is collected, on one embodiment, between the supraventricular electrode and the atrial electrode connected to it, and, on the other embodiment, between the supraventricular electrode and a second atrial electrode that is not connected to the supraventricular electrode. Even if the signal quality is improved with this lead structure, this lead structure is a non-standard model, therefore it cannot be implemented easily and widely by conventional techniques.
Other techniques have been proposed to collect atrial activity with monobody standard leads including one (or two) distal electrode(s) for monopolar detection (or bipolar detection, respectively) of a ventricular signal, a ventricular coil forming a defibrillation electrode, and a coil positioned mainly in the superior vena cava (SVC), in the vicinity of the atrium. The difficulty with this technique is that the SVC coil is not an electrode suitable for atrial detection, and it does not allow for a proper collection of atrial cardiac activity signal, especially because this electrode is electrically floating and delivering a highly noisy signal.
In particular, the atrial signal collected on the SVC coil (or more precisely, a monopolar detection between the SVC coil and the generator housing) is distorted by interference from the ventricular signal, which is often greater in amplitude than the atrial signal.
Techniques have been proposed to discriminate the atrial and ventricular components and to extract the atrial signal having a lower amplitude than the ventricular signal.
U.S. Pat. No. 5,776,072 describes one proposal in which, after signal detection and appropriate filtering, a transfer function of the ventricular channel signal compared to the signal on the combined channel (atrial+ventricular) is estimated. The estimated transfer function reflects the contribution of the R-wave to the signal on the combined channel. The application of the estimated transfer function to the signal of the combined channel provides a resultant signal that is subtracted from the signal collected on the combined channel to obtain a residual signal corresponding to an approximation of the P wave.
U.S. Pat. No. 5,885,221 (assigned to Cardiac Pacemakers Inc.) is another proposal that describes a technique, after estimating the transfer function, for calculating a convolution between the combined signal and the transfer function, thereby to obtain the contribution of the ventricular signal. This ventricular signal contribution is then removed from the combined signal to deliver an estimate of the atrial signal.
In these two prior art techniques, the discrimination between atrial and ventricular components is obtained by identifying and subtracting the R-wave signal component from a combined signal measured by a bipolar detection between the right ventricular (RV) coil electrode and the SVC coil. The calculation performed is relatively complex, and the results of these techniques have never been published. Therefore, it is unknown whether, in practice, these techniques produce a satisfactory estimate of the P wave, and whether they are clinically effective.